1. Field of the Invention
The present disclosure relates to a sheet supply device for supplying a sheet like a paper, and an image forming apparatus.
2. Description of the Related Art
Sheet supply devices are widely used by the image forming apparatus like copying machines, facsimile machines, scanners, and multifunction apparatus. Using the sheet supply device, the sheets like papers to which an original image is transferred are conveyed one by one to a position to form the image thereon, whereby the image forming apparatus can print each image successively.
It is configured in the sheet supply device that a sheet bundle is allowed to touch a pickup roller, in order that a plurality of sheets stacked onto a tray is fed one by one. A thickness of sheet bundle depends on the number of sheets stacked on the tray. Accordingly, a distance between a sheet stacking surface and the pickup roller should be adjusted according to the thickness of sheet bundle, in order to allow the sheet bundle to properly touch the pickup roller. A lifting plate liftably mounted to the sheet stacking surface can be used for such adjustment. In the sheet supply device, the sheet bundle on the lifting plate is pushed up by the lifting plate, and the sheet bundle is allowed to touch the pickup roller, and then the sheet touching the pickup roller is sent out one by one.
In the above sheet supply device, when the temperature or moisture environment changes in the image forming apparatus or when members are worn out by use, there is a possibility that sheet feeding troubles like no paper feeding and double feeding occurs due to the change of the ideal conditions for feeding the sheets. As the apparatus that has settled such problem, there is an image forming apparatus wherein the speed of the pickup roller is adjusted based on a comparison result between a conveyance time for conveying a preceding sheet and a standard conveyance time recorded in advance. In this image forming apparatus, when the sheet conveyance time gets long due to the occurrence of skid, the skid of a following sheet can be suppressed by reducing the speed of the pickup roller.
It sometimes occurs that user of the image forming apparatus wishes to use a different kind of sheet instead of the sheets set in the sheet supply device. At this time, he uses his desired sheets without replace all the sheets set in the sheet supply device with his desired sheets but adding his desired sheets to the sheets set in the sheet supply device. In order not to include his undesired sheet in the output sheets on such condition, there is an image forming apparatus wherein the added sheets and the preset sheets are managed separately.
When all the sheets on the tray are sent out by the sheet conveying, a sheet set sensor for detecting the sheets on the tray does not detect any sheets (a detection state of no sheet), and the sheets must be set. In the latest image forming apparatus, when the state of no sheet is detected, the user is not requested to set the sheets, but a display provided to the apparatus displays a remaining amount of sheet. In some of those apparatus, when the remaining amount of sheets gets small, the display displays a warning message to draw an attention from the user. By promoting the user to set the sheets just before the state of no sheet, the running-out of sheets can be prevented in the middle of the job processing.
As described above, in case of the image forming apparatus for promoting the user to set the sheets just before the state of no sheet, the user confirms the message and sets the same kind of sheets as necessary. In this case, if the sheet bundle is remaining on the tray, it occurs that another sheet bundle is added on the remained sheet bundle in such state. In case of such sheet addition, an air layer is produced at a paper adding position, that is, a position between a highest surface of the remaining sheet bundle and an undermost surface of the added sheet bundle. The frictional force between those surfaces becomes small as compared with the frictional force between the adjacent sheets. Therefore, when the sheet near and on the paper adding position is sent out, it sometimes occurs that one sheet is not fed out, but plural sheets on the paper adding position are separated from the paper adding position, and they are sent out by the pickup roller. In this case, the feeding failure occurs.
The feeding failure cannot be eliminated even if the foregoing method for adjusting the speed of the pickup roller is applied, because the preceding sheet has been fed normally. In case of the method for managing the added sheets and the preset sheets separately, the paper adding position can be managed, but the occurrence of the feeding failure cannot be recognized, and the feeding trouble cannot be settled. Moreover, the method of managing the added sheets separately assumes that the different kind of sheet is added, and it is not configured so as to separately manage the same kind of sheet, therefore it is difficult to use the method.